Resistor compositions



Feb. 1966 L. c. HOFFMAN 3, 3

RES ISTOR COMPOS ITIONS Filed Sept. 20, 1962 2 Sheets-Sheet 1 FIG.!

I00 PALLADIUM INVENTOR LEWIS I c. HOFFMAN ATTORNEY Feb. 1, 1966 c. HOFFMAN 3,232,886

' RESISTOR COMPOSITIONS Filed Sept. 20, 1962 2 Sheets-Sheet 2 FIG. 2

NOISE VOLTAGE- #V/ VOLT IN A DECADE '50 -20 -IO 0 H0 +20 +30 NOISE INDEX DECIBELS IN A DECADE INVENTOR LEWIS C. HOFFMAN ATTORNEY United States Patent 3,232,886 RESISTOR COMPOSITIONS Lewis Charles Hotfman, Wycliffe, Wilmington, Del., as-

signor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Sept. 20, 1962, Ser. No. 224,915 2 Claims. (Cl. 252-514) This invention relates to resistor compositions; and more particularly, it relates to conductive glass frit-precious metal resistor compositions suitable for production of electrical resistors having an improved low current norse.

Precious metal resistor compositions containing a glass frit binder which may be applied and fired on a ceramic base to produce an electrical resistor have been known for some years. Such compositions are described in US. Patent Nos. 2,924,540, 3,052,257 and 2,950,995 and have been used commercially to some extent. Such previously known resistor compositions were not entirely satisfactory because resistors produced therefrom had an undesirably high noise character.

It is an object of this invention to produce new and improved glass frit precious metal resistor compositons that are suitable for use in producing electrical resistors of low noise rating.

It is another object of this invention to provide an improved method for the production of glass frit precious metal resistor compositons.

It is yet another object to produce electrical resistors of the ceramic base type which have a reduced noise rating.

Other objects will appear hereinafter.

The objects may be accomplished by mixing 20 to 49 percent by weight of finely divided glass frit with 80 to 51 percent of finely divided low oxide content palladium and silver in which the ration of palladium to silver is between 55:45 to 45:55. I

The invention will be described with reference to the accompanying drawings in which;

FIG. 1 is a ternary diagram of the palladium-silverfrit ternary mixtures showing in area ABCD the essential ternary mixtures to produce the resistor compositions of this invention;

FIG. 2 is a noise rating conversion diagram showing the straight-line relationship between noise voltage in microvolts per decade and noise index in decibles per decade.

When a steady direct current is passed through a resistor, fluctuations in the amount of current being drawn occur due to sporadic, very small changes in the resistor current path. These fluctuations give rise to potential differences which find their way into other parts of the circuit as spurious signals (voltages) or noise. Even continuous, current paths, such as those found in wire wound resistors exhibit some noise probably due to localized composition or textural variations in the poly! crystalline metal structure. The more granular a resistor, the higher its current noise. Thus, carbon composition resistors and previously known palladium-silver glaze resistors have had a relatively high noise factor.

Noise is measured by comparing the output from the resistor under D.C. load with the output of a signal gen-.- erator over a decade of frequency. The Bureau of Standards has developed a special circuit which is incorporated in the resistor noise test set model 315 manufactured by Quan-Tech Laboratories, Inc., Boonton, NJ. Noise is expressed by the ratio of the root means square noise voltage in microvolts to the applied DC. voltage in volts expressed in decibels, when the associated pass band is one frequency decade. Noise in microvolts 3,232,886 Patented Feb. 1, 1966 iCe per volt and decibels per decade are easily interconverted, as shown in FIG. 2 of the drawings; but in this memorandum all noise figures will be in decibels per decade.

It has now been found in accordance with this inventionthat superior resistor compositions, from a standpoint of low noise characteristics, can be prepared from finely divided palladium, silver and glass frit if the frit composes less than 49% of the total weight of these constituents, if the ratio of palladium to silver is between 45%:55% and 55%:45%. Moreover, the content of combined oxygen in the palladium should be between 0.1% and 1%.

Referring to FIG. 1 of the drawings, the area ABCD of the diagram discloses the area of lowest noise in the system glass frit-palladium-silver. The resistance, in ohms per square per mil thickness of resistance coating, and the current noise in decibles per dcade are given for 21 points in the ABCD area.

When the frit content of the compositon is greater than 50% by weight, the noise rapidly rises to above 20 decibels per decade which is considered to be about the borderline of acceptability in resistors of this type. The very best results, from a standpoint of noise, are obtained when the palladium and silver are present in about 5050 by weight. This is clearly observable on the ternary diagram. The reason for this optimum ratio of PdzAg is believed to have a relationship to the strong alloying tendency between Pd and Ag because of their closed similarity in atomic radii. When such alloys are formed, silver can contribute one electron to the d shell of Pd, thus completing it. This is a thermodynamically more stable situation than exists when either metal is alone, and this increased stability is the driving force for the alloying. The most stable alloying situation exists when each Pd atom shares an electron with a Ag atom. The most stable alloy is therefore formed at an equal atomic ratio and since Ag and Pd have almost the same molecular weights, the weight ratio is also close to 1:1. It is important to maintain the ratio of Pd to Ag in the composition to between 55:45 and 45 :55.

Another requirement for low noise resistors is to start with palladium containing less than about 1% combined oxygen. Pd powder when heated will at first oxidize, oxidation beginning at about 350 to 400 C.; and upon further heating to a higher temperature, 650 to 750 C., some of the formed PdO will decompose with oxygen evolution. If the Pd in the resistor composition contains too much 0 i.e., more than about 1%, the firing of the composition to fuse the glass frit binder will not remove sutficient 0 from the Pd to produce a resistor composition that will have a sufficiently low noise index. In order to have a satisfactory noise index, the content of combined 0 in the PdPdO should be less than about 1% by weight.

A certain small percentage of PdO in the Pd is, however, essential. If oxygen-free Pd is used in the composition and the composition is fired in an oxygen-free atmosphere, e.g. argon, the fired resistor will not have reproducible resistance values and will have objectionable high positive temperature coefiicients of resistance. The Pd should therefore contain an essential minimum of combined oxygen of about 0.1%.

The Pd should be finely divided to between about 0.5 to 50 microns in diameter and may be obtained from any source of production that will produce a Pd containing not to exceed 1% of combined oxygen. A very desirable source of Pd is that produced by the reaction of hydrizine sulfate with PdCl in the presence of less than 12 cc. of concentrated aqueous ammonia per gram of palladium.

The silver powder used in the low noise resistors of this invention may be produced by chemical precipitation or by mechanical means as long as the particle size is of the order of 0.1 to 50 microns in diameter. There is nobeneficialetrect to be gained by after treatment nor deleterious effect as a result of oxidation. Any silver oxide present will decompose at such low temperature that it is substantially absent at the firing temperature of the glazed resistor.

Insofar as noise factor is concerned, the composition of the glass frit is not critical. Borosilicate glasses such as lead borosilicate, cadmium borosilicate and similar glasses are suitable for the production of the low noise resistors of this invention. If, however, it is desirable to produce low drift and low TCR (temperature coefficient of resistance) resistors, then it would be advisable to use the Na O-ZnO-Al O B O -SiO -ZrO frits disclosed and claimed in my copending application Serial No. 225,121, filed of even date herewith.

The finely divided dry mixture may be made into a paste or liquid by the addition of a vehicle in a known manner. Fifteen percent to sixty percent by weight of an inert liquid, for example, water, methyl, ethyl, propyl,

butyl orhigher alcohols, the corresponding esters such as acetates, p'ropionates, the terpenes and liquid resins such as pine oil or alpha or beta terpineol and the like may be used to prepare a suitable consistency for application purposes. The vehicles may contain or be composed of volatile liquids to promote fast setting after application, or they may contain waxes, thermoplastic resins, or wax-like materials which are thermofluid by nature whereby the composition may be applied to a ceramic insulator while at elevated temperature to set upon contact with a cold ceramic base. I

The following example is given to illustrate in detail a complete procedure for the production of a low noise resistor in accordance with this invention.

Thirty parts, by weight, of finely divided silver, 30 parts of finely divided palladium containing less than 0.4 part combined oxy'gen and 40 parts of a glass frit consisting of 33.1% ZnO, 24.5% SiO 26.7% B 3% A1 0 8.7%Na O and 4% ZrO were dry mixed until homogeneous and then dispersed in 50 parts by weight of "an organic medium composed of 8% ethyl cellulose and 92% butyl cellosolve acetate to make a paste suitable to be squeegeed through a screen stencil.

A inch x 1 inch x inch alumina plate was first provided with spaced dried silver terminals. The squeegee resistor composition was stencilled over the terminations so as to provide an area of overlap and an area of film having a thickness of about 1 mil in direct contact with the alumina. The plate was then fired at a temperature of 760 C. for 12 minutes. Wires were soldered to the silver terminal overlap. This resistor when tested in the resistor noise test set model 315 manufactured by Quan-Tech Laboratories, Inc. of Boonton, N.J., had a noise index of 30.6 decibels per decade.

Throughout the specification and claims, any reference to parts, proportions and percentages refers to parts, proportions and percentages by weight unless otherwise specified.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

I claim:

1. A resistor composition consisting essentially of to 49 percent by weight of finely divided glass frit and 51 to 80 percent of finely divided palladium and silver in the weight ratio of palladium to silver of 55:45 to :55, said palladium containing between 0.1% and 1% of combined oxygen and consisting of particles of from 0.5 to microns in diameter, and said silver consisting of particles of from 0.1 to 50 microns in diameter.

2. A resistor composition as defined in claim 1 containing between 15 and by weight of the composition of a liquid inert vehicle.

References Cited by the Examiner UNITED STATES PATENTS JULIUS GREENWALD, Primary Examiner. 

1. A RESISTOR COMPOSITION CONSISTING ESSENTIALLY OF 20 TO 49 PERCENT BY WEIGHT OF FINELY DIVIDED GLASS FRIT AND 51 TO 80 PERCENT OF FINELY DIVIDED PALLADIUM AND SILVER IN THE WEIGHT RATIO OF PALLADIUM TO SILVER OF 55:45 TO 45:55, SAID PALLADIUM CONTAINING BETWEEN 0.1% AND 1% OF COMBINED OXYGEN AND CONSISTING OF PARTICLES OF FROM 0.5 TO 50 MICRONS IN DIAMETER, AND SAID SILVER CONSISTING OF PARTICLES OF FROM 0.1 TO 50 MICRONS IN DIAMETER. 